Some selected highlights

GO 11113: Binaries in the Kuiper Belt: Probes of Solar System Formation and Evolution

A composite of HST images of the Kuiper Belt binary, WW31

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band
stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun
(see David Jewitt's Kuiper
Belt page for details). Over 500 KBOs are currently known out of a population of
perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known
KBOs are binary (including Pluto, one of the largest known KBOs, regardless of
whether one considers it a planet or not). This is a surprisingly high fraction, given
the difficulties involved in forming such systems and the relative ease with which
they can be disrupted. It remains unclear whether these systems formed from single
KBOs (through collisions or 3-body interactions) as the Kuiper Belt and
the Solar System have evolved,
or whether they represent the final tail of an initial (much larger) population of
primordial binaries. This proposal will use WFPC2 imaging of known KBOs to identify
new binary systems.

GO 11211: An Astrometric Calibration of Population II Distance Indicators

Measuring trigonometric parallax

Trigonometric parallax measurement remains the fundamental method of determining
distances to astronomical objects. The best ground-based parallax measurements
can achieve accuracies of ~1 milliarcsecond, comparable with the typical accuracies
achieved by the ESA Hipparcos astrometric satellite. This level of accuracy allows us
to obtain reliable distances and luminosities for main sequence stars, subgiants, red giants
and even a number of metal poor subdwarfs. However, with an effective distance limit of
100-150 parsecs, the sampling volume includes less than a handful of rarer,
shorter-lived celestial objects. In particular, there are no RR Lyraes or Cepheids, two of the
principal calibrators in the extragalactic distance scale. There is only one
instrument currently available that can achieve astrometry of higher accuracy - the Fine
Guidance Sensors (FGS) on HST. The present team used the FGS to measure a parallax of
3.82+/10.2 milliarseconds for RR Lyrae, the nearest star of its type. this corresponds
toa distance of 262 parsecs. The present program aims to improve the calibration
by extending observations to four more
relatively nearby RR Lyraes (XZ Cyg, UV Oct, RZ Cep and SU Dra) and two Pop II Cepheids
(Kappa Pav and VY Pyx).

Gravitational lensing supplies a powerful method of tracing the mass distribution
in galaxy clusters; at the same time, the amplified the light from
background galaxies provides a means of probing the early stages of galaxy formation.
These measurements are particularly effective when X-ray imaging data are also
available, allowing direct measurement of the mass density and distribution of the
hot intracluster medium. This snapshot proposal aims to use the Wide Field Camera on ACS
to observe the central regions of
low redshift (0.15 < z < 0.3) clusters with the requisite Chandra observations.
The HST images will allow the resolution of lensed arcs in the cluster cores
(due to strong lensing) and characterisation of weak-lensing distortions of
the image profiles of faint background galaxies.
The frequency and detailed distribution (size, multiplicity, redshifts) of the
strong lens systems sets
strong constraints on the total mass content, and its structure, in the
central regions of low-redshift clusters. Those results, in turn, constrain
cluster evolution, and offer insight into likely schemes for studying dark energy
at higher redshifts.

GO 11513: The afterglow and host galaxy of GRB 080319: the first "naked eye" burst

The life history of a gamma-ray burst

Gamma ray bursts are described colloquially as the biggest bangs since the Big Bang. Originally
detected by US spy satellites in the 1960s, these short-lived bursts of high energy radiation
resisted characterisation for over 30 years. It is only within the last decade that the
Galactic vs. Extragalactic debate on their origins has been setled in favour of the latter.
Generically, gamma ray bursts are believed to originate in the death throes of an
extremely massive star, as it collapses to form either a black hole or a highly magnetised
neutron star. Most occur at moderate to high redshifts, and the optical flashes reach magnitudes
between 12th and 15th at their peak. However,
GRB 080319B , detected by the SWIFT satellite at
06:12 UT on March 19th 2008 (one of four bursts detected on that date),
shattered all previous records. Optical imaging by the "Pi of the sky" wide-field
monitoring survey shows that the source
reached a peak brightness
of mV~5.5 , remaining above the naked-eye threshold for ~30 seconds.
The parent galaxy lies at redshift z~0.94, so the peak brightness was close to 1017
solar luminosities. The burst has faded significantly, but may still be visible to HST, which
can also resolve the detailed structure of the host galaxy.